Many small computers utilize an audio-frequency magnetic tape recorder as an inexpensive auxiliary data storage device. In order to utilize an audio tape recorder in this manner, an interface circuit is provided between the computer and the tape recorder which converts digital signals produced by the computer into audio-frequency tone signals that can be recorded on magnetic tape by the recorder audio circuitry. Typically, a digital signal which represents a logical "one" is converted to an audio tone of a predetermined frequency. A digital signal which represents a logical "0" is converted to an audio tone of a different predetermined frequency. The audio tones produced by the tape recorder interface can then be conveniently stored on inexpensive audio magnetic tape.
Subsequently, the recorded information can be retrieved from the tape by "playing back" the tape and reconverting the resulting audio tones into digital signals suitable for use by the associated computer.
In order to achieve reasonable information recording density on the magnetic tape only a single cycle of each audio frequency is used to represent a digital "bit". Therefore, the serial stream of audio information which is retrieved from the tape consists of a frequency-modulated waveform containing a pattern of various frequencies corresponding to the digital pattern of "1"s and "0"s in the information originally stored on the magnetic tape.
While audio tape storage is inexpensive and reliable, a synchronization problem can occur because some audio tape recorders cause an inversion of the audio signal when a stored signal is retrieved from the tape. Other tape recorders do not invert the signal when the stored information is retrieved. Whether an inversion occurs depends on specifics read circuitry in the tape recorder and may vary from brand to brand and among different models in the same brand of tape recorders. An inversion can prevent the computer-tape recorder interface from properly synchronizing to the audio signal output from the cassette recorder so that a conversion can be performed between the audio signals on the digital levels required by the associated computers.
One simple prior art solution to the inversion problem is an inverting switch. When an inverting switch is used the recorder interface first attempts to synchronize to the incoming signal and if synchronization does not occur the switch must be manually moved to invert the waveform and allow synchronization to take place. This approach is inconvenient and, in some cases, the operator of the computer may not be aware of the source of the problem and, therefore, may not be able to correct it.
It is, therefore, an object of this invention to provide cassette interface synchronization circuitry which can synchronize to a waveform whether the waveform is inverted or not.
It is a further object of the invention to perform synchronization with simplified circuitry.
It is a still further object of the invention to perform the synchronization automatically without the need for manual operator intervention.